Car seat safety is my passion as I have learned a lot from the car seat technicians at carseat.org. So I quickly wanted to give all parents a quick "to do" list as it relates to your child's car seat.
1. If your car seat is loose and can be moved from side to side after it's buckled into your car, most likely it's too loosely installed. Follow this tip. Put your knee on the car seat and put as much weight as you can onto it so much as to compress your vehicle's seats, then pull any slack out of the seat belt. If your child's car seat is rear facing and it is hard to climb in your child's seat to do this, try your best to apply pressure with your body in any other way you can manage. Visit carseat.org for any further fine tuning. Ex. They will tell you if you need to and how to use a locking clip, or how to pull the slack out of your seat belt, or depending upon which locking mechanism your car's seat belts are designed with, how to lock your seatbelts so that your car seat is secured. Car seat belts should lock and not have any give (namely, go anywhere) if they are locked. Child car seats need the vehicle's seat belts to be already "locked" upon impact should you have a collision. The locking mechanism is the mechanism that locks the seat belt against a person's chest when in a collision. At other times the seat belt has "give" if you want to lean forward or stretch out the seat belt with your hands, for example purposes only.
2. If your child is forward facing, make sure that the harness straps are either level with your child's shoulders or higher on the next highest notch up. For example, my 4 yr. old's harness straps are at least 1 to 2 centimeters above her shoulder level, because the only other closest notch is below her shoulder level. When harness straps are below shoulder level for forward facing children, in a collision, the harness straps can compress their spine and hurt them rather than help them.
If your child is rear facing, alternatively, the harness straps must be below the child's shoulder level. This helps in securing their body from ejection in a crash, and it does not cause spinal compression because the rear facing car seat is reclined at least 30 degrees so that in a collision, your child's body rides up the back of the car seat shell. You can see an example of how the rear facing child's body moves in a collision on youtube.com.
3. Usually, on most car seats, there are 2 belt paths. One for forward facing and one for rear facing. It is important you are using the correct one. Read your manual and visit carseat.org for help. Some car seats have other features as well. You need to read the manual or visit carseat.org to see if you are using those other features correctly.
4. If you are forward facing and have tether capability in your vehicle, use it. It will prevent your car seat and thus child from possibly hitting the seats in front of them possibly reducing neck and arm and leg and head injuries from hitting the seat back in front of them.
5. Any other questions, visit carseat.org.